Numerous signal processing applications, such as adaptive antennas and radar, require the use of electronic delay lines. A number of types of crystal delay lines have heretofore been used for this purpose. Adjustable time delays have been provided by switching between devices each having a different fixed delay or by switching between taps of a single device having a multitap delay line. These delay lines suffer the disadvantage that the delay is incremented in finite steps which results in poor signal resolution or an exceedingly complex device with a large number of taps. A further type of crystal delay line has used dispersive transducers to vary the delay along the crystal as a function of the input signal frequency. A delay line of this type requires frequency converters at the input and output transducers. This approach does not work well with a large percentage bandwidth signal or with narrowband signals which can be received over a wide tunable bandwidth. A further approach has been the use of a controlled poling field across the propagation path of a surface acoustic wave device to vary the phase velocity of the surface waves. This approach, however, has provided only a very limited variation in time delay even with a strong poling field.
Therefore, there exists a need for a continuously variable electronic delay line which has a relatively large bandwidth, a substantial variation in time delay and continuous adjustment over the time delay range.